数字水印技术的DEM版权保护适用性研究

Applicability of Watermarking Techniques to DEM

数字水印技术可为DEM的版权保护提供有效的解决方案。探讨了DEM数字水印技术的要求,提出"近无损"性是DEM区别与普通图像数字水印的主要特征。即含水印DEM除与普通数字图像一样须满足水印的不可见性外,还应当满足DEM精度"近无损"和应用"近无损"要求。通过实验分析了考虑视觉模型的DCT分块数字水印算法、DWT数字水印算法对DEM的适用性问题。结果显示,水印对DEM精度与地学基本应用有一定影响,以图像视觉质量为优劣标准的数字水印算法并不一定能满足DEM的精度和应用要求。

The demand for DEM data has dramatically increased mainly due to the large number of possible applications capable to exploit DEM data.As in many other fields,with the Internet has become preferred and effective means of data exchange,the online purchasing and distribution of DEM can now be performed relative easily.It follows that the copyright protection is increasingly becoming more and more important.Watermarking represents a potentially effective tool for the protection and verification of ownership rights in DEM.But before applying watermarking techniques developed for multimedia applications to DEM,it is important that the requirements imposed by DEM are carefully analyzed to investigate whether they are compatible with existing watermarking techniques.On the basis of these motivations,an overview of watermarking is given,and the contribution of this work is twofold:①Assessment of the requirements imposed by the characteristics of DEM on watermark-based copyright protection is compared with usual digital images.In the multimedia field,watermark must be transparent to the human visual system.While in the DEM context,this requirement can be rephrased as 'not altering the elevation precision and the results of application to an extent of DEM besides transparency'.This is called near-lossless requirement in this paper.It is worth noticing that near-lossless is considered more necessary than robustness requirement.That is because almost nobody would attack DEMs with degraded quality.Adaptivity is another characteristic of DEM watermark technique.Generally speaking,DEMs required high precision could be embedded in less watermark energy.While DEMs required low precision are on the contrary.Since different precision standards are established for different landforms of DEM,watermark energy embedded should also change adaptively to landforms.It ensures that the watermarked DEM meets accuracy requirements of DEM as well as watermark energy could be inserted as much as possible.In addition,efficiency of watermark algorithms should also be considered.②Discussion of a case study where the performance of two popular,state-of-the-art watermarking techniques is evaluated by the light of the requirements at the previous point.One is based on block-based DCT techniques using Watson perceptual model,the other is based on DWT techniques using PWM perceptual model.The two algorithms are applied to a 512×512 DEM of Loess Plateau with a horizontal grid spacing of 5m.The watermark is a 64×64 binary image.In the experiment,watermarked DEMs are ensured transparent to the human visual system.In order to evaluate the impact of the watermark presence on DEM,we considered histogram,slope,slope of slope,contours,collection waterlines and terrain structure lines derived from the two watermarked DEMs.It has been shown that the presence of the watermark has almost no impacts on histogram.However,the results of the applicative tasks in digital terrain analysis are heavily affected not only in the human vision but also in values.As for the DWT algorithm,from which contours,collection waterlines and terrain structure lines derived all exhibit poorer precision.Obviously,watermarking techniques evaluated by human visual system don't always meet the requirements of elevation precision and applications of DEM.Watermark embedding should be very limited on avoiding DEM quality degradation.In the further,work will aim at the development of watermarking techniques specifically designed for DEM,so as to minimize the impact on the applications.